Promote PendingDeprecationWarnings to DeprecationWarnings

[hugovk]

Fixes #1773.

[hugovk]

Merge conflicts resolved!

[jodal]

Thanks! I plan to merge this as soon as we've thrown out the last of the old deprecated fields, ref. #1083 and #1461.

[codecov]

Codecov Report

Merging #1786 into develop will decrease coverage by 1.16%.
The diff coverage is 87.5%.

@@             Coverage Diff             @@
##           develop    #1786      +/-   ##
===========================================
- Coverage    82.02%   80.85%   -1.17%     
===========================================
  Files           77       77              
  Lines         6319     6325       +6     
===========================================
- Hits          5183     5114      -69     
- Misses        1136     1211      +75

Impacted Files	Coverage Δ
mopidy/core/tracklist.py	96.41% <100%> (+0.9%)	⬆️
mopidy/core/playback.py	76.97% <71.42%> (-14.78%)	⬇️
mopidy/mpd/protocol/playback.py	91.66% <75%> (-7.5%)	⬇️
mopidy/mpd/protocol/status.py	83.19% <0%> (-15.13%)	⬇️
mopidy/internal/deprecation.py	95% <0%> (-5%)	⬇️
mopidy/core/actor.py	90.47% <0%> (-4.09%)	⬇️

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[hugovk]

Got a bit stuck here, seems the new ones are not quite drop-in replacements! Could I hand this PR over?

[jodal]

Replaced by #1854. See the description there.